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How would I should I shorten the following piece of code?

I have the following piece of code. It is simple and does the job... But it's too lengthy for my liking.
if ( isInputValid( userInput ) ) {
return userInput;
} else {
console.log('User Input is Invalid!');
}
Normally, I would just be able to use a ternary operator. However, in this case, I can't.
// not possible, invalid
isInputValid( userInput ) ? return userInput : console.log('User Input is Invalid!');
But, is there another way to shorten and further simplify the if and else statement? Anything like the fat arrow syntax, etc?
Much appreciated.

Seems like you have a few options. First, since you appear to be returning from a function, you could shorten your initial code by simply omitting the else part.
if ( isInputValid( userInput ) ) {
return userInput;
}
console.log('User Input is Invalid!');
Now you could still use a ternary operator if you want, you just need to return the result of the ternary operator itself.
return isInputValid(userInput) ? userInput : console.log('User Input is Invalid!');

Building on #Nick's answer, a good way to shorten this code would be to simply do the following.
if ( isInputValid( userInput ) ) return userInput;
console.log('User Input is Invalid!');
Being able to remove the braces, due to it being a single statement, followed by removing the else as console.log() will really only be executed if the previous statement is false anyway. So, it renders the else unnecessary.
Short and simple, beautiful!

Conditional function call without if

I used the following code a lot in my work
if (people === 'someone') {
doSomething();
}
Is there any shorter way to do it ?

You can write like this:
people === 'someone' && doSomething()
It means if the left side is true, then right side will be take into action. If the left side is false, the statement end.
Note: I agree with meagar , if is more readable.
And in case you interested in how the logic operator work, see this https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_Operators

You can use ternary operator. A Standard Ternary is simple, easy to read, and would look like this:
$value = ($people === 'someone') ? doSomething() : 'nothing';
or
echo ($people === 'someone') ? doSomething() : 'nothing';

Do I need to use an "=" when using the ? : javascript shortcut?

Can someone tell me if this is valid javascript to do this:
if (wf.statusId == Status.Dirty) {
wf.createdDate
? promises.push(self.wordFormUpdateSubmit(wf, key))
: promises.push(self.wordFormAddSubmit(wf, key));
}
Would there be cases where this would not work correctly if createdDate was not defined?
Here's what this replaced:
if (wf.statusId == Status.Dirty) {
if (wf.createdDate) {
var updatePromise = self.wordFormUpdateSubmit(wf, key);
promises.push(updatePromise);
} else {
var addPromise = self.wordFormAddSubmit(wf, key);
promises.push(addPromise);
}
}
Also a related question. Would it be possible to use the same syntax with ? : to replace the need for the if () { } construct ?

Using = with ternary operator is not mandatory unless expr1 and expr2 are returning something and you want to save it in some other variable.
In your case, unless promises.push(self.wordFormAddSubmit(wf, key)) and promises.push(self.wordFormUpdateSubmit(wf, key)) are returning something that you want to save it a variable, there is no need for a =.

if (wf.statusId == Status.Dirty) {
promises.push(self[wf.createdDate ? 'wordFormUpdateSubmit' : 'wordFormAddSubmit'](wf, key));
}

wf.createdDate only true if it is not undefined or have value. so this should work fine.
if (wf.statusId == Status.Dirty) {
wf.createdDate
? promises.push(self.wordFormUpdateSubmit(wf, key))
: promises.push(self.wordFormAddSubmit(wf, key));
}
Regards
Mk

This is an example of a ternary statement, using the conditional (ternary) operator, which by definition replaces an if...else construct.
From MDN:
The conditional (ternary) operator is the only JavaScript operator that takes three operands. This operator is frequently used as a shortcut for the if statement.
[source]
Both of your code samples would work the same way, ie if createdDate was undefined the second expression of your ternary statement would run (the line after the :) just like the else block of your if...else construct would run.
The title question seems somewhat unrelated, in that you would only need to use = if you wanted to save a reference to something. In this case, it does not appear that you do.
As for your follow-up question, plenty of people use ternary statements for small checks and tasks that fit on one or a few lines (I personally don't because I prefer the readability of if...else), however anything even moderately complex can quickly make your code hard to read and understand. But yes, technically, ternary statements can replace if...else blocks.

regarding the second question:
assuming you don't need to save the results in a variable, you can do:
promises.push(wf.createdDate?
self.wordFormUpdateSubmit(wf, key) :
self.wordFormAddSubmit(wf, key);
);
and even this works:
promises.push(
(wf.createdDate? self.wordFormUpdateSubmit:self.wordFormAddSubmit)(wf, key)
);

If true … else Shorthand

I know that the normal use case for this if statement is e.g.
var string = boolean ? "this" : "that";
I use jhint in my editor and when i try something like
boolean ? array.push("this") : array.slice("that",1);
jshint throws (W030) "Expected an assignment or function call and instead saw an expression"
So far the code always worked fine but maybe i was just lucky.
So my question is, why should i not use this pattern and what would be an alternative? Because writing
if(boolean){
array.push("this");
} else {
array.splice("that",1);
}
for such short instructions really give me the creeps.
thank you.

It is possible to wrap the ternary operator inside the void operator like this:
void(cond ? expr1 : expr2);
This achieves the desired result and passes JSHint. See JSFiddle and click JSHint button.
However, I recommend the following syntax:
if (cond) {
expr1;
} else {
expr2;
}
Because it is more readable. Just because JavaScript lets you do strange things does not mean that you should.

What it complains about is you misappropriating the conditional operator. It is an operator not a control structure. So it lives in the category of such things things as +,-,*,/. That means you expect the first operand to be a boolean and the second and third to yield a return value.
The whole thing is meant to be short for
if (boolean) {
string ="this" ;
} else {
string ="that";
}
It wants to return a value (which it can't in your case) and it expects you to use that value (which you don't). So the tenary if is not the thing to use for your case and as a result makes it far less readable.

You are using side effects in expressions to execute logic.
Indeed not very friendly code. It will work.
Just rewrite to distinct logic from expressions.

You can circumvent the jshint message using:
void(boolean ? array.push("this") : array.slice("that",1));

If you really want to use the ternary operater for this kind of operation in a clean way, then you can do it like this:
array[cond ? 'push' : 'slice'](cond ? "this" : "that", cond ? 1 : undefined);
or
array[cond ? 'push' : 'slice'].apply(null, cond ? ["this"] : ["that", 1]);
But anyway you may prefer a boring if statement.

Return of function in ternary comparison

simple code but answer not found (maybe I haven't looked deep enough since my main language isn't english..)
getDeviceInfos(deviceIP) ? displayDevice(**return of getDeviceInfos function**) : dead.push=deviceIP;
If getDeviceInfos returns something else than 0 (ie. it == true), i want the return value to be the argument in displayDevice call.
Is there any way to do it or I need to write a "regular" comparison ?
Thanks

You can do the following, but it would need a var to avoid a global variable. As a result, it's not really a straight expression.
var info;
(info = getDeviceInfos(deviceIP)) ? displayDevice(info) : (dead.push = deviceIP);
While this wasn't your question, you can (and perhaps should) do something similar by caching the result with a standard if-else statement, either when you var info, or in the if itself:
var info;
if (info = getDeviceInfos(deviceIP)) {
displayDevice(info);
} else {
dead.push = deviceIP;
}
Provided you're not using the value produced by the ternary expression, I would recommend the latter approach for readability.

First create a variable, then perform the assignment and compare like so -
var a; // <-- A variable.
(a = getDeviceInfos(deviceIP)) ? displayDevice(a) : dead.push = deviceIP;

Based on your update in the comments sections (and for readability's sake), I'd suggest making it a two step check:
var deviceInfo = getDeviceInfos(deviceIP);
(deviceInfo !== 0) ? displayDevice(deviceInfo) : dead.push=deviceIP;
That is a more accurate check of the condition and is easier to read.

Conditional Operator
Multiple ternary evaluations are also possible (note: the conditional operator is right associative).
var hadRelations = false;
var isSure = false;
var presidentQuote = hadRelations ? "Had relations" : isSure ? "Did not have relations" : "I admit";
console.log( presidentQuote ); // Prints "I admit" in the console

I moved it into the function itself..
function getDeviceInfos(pIp)
{
//code
result ? displayDevice(result) : dead[dead.length]=pIp
}

I know this isn't strictly speaking an answer your question, but I'd strongly recommend considering not using the ternary syntax, ever. When writing code, always balance what is the easiest to read and maintain with what is compact, efficient, and cool.
In practice, ternary syntax isn't significantly faster than an if statement, so there's generally no "efficiency" grounds to use it.
Compare:
var info;
(info = getDeviceInfos(deviceIP)) ? displayDevice(info) : (dead.push = deviceIP);
with ...
var info = getDeviceInfos(deviceIP);
if (info) {
displayDevice(info);
} else {
dead.push = deviceIP;
}
Sure, the if-then style is less compact, but it's much easier to understand, debug and extend.
In short, there's almost never a reason to use ternary syntax: it's generally harmful to code quality. Avoid.